During the 1880's, an infrared detector called the bolometer was developed. The bolometer operates on the principle that the electrical resistance of the bolometer material changes with respect to the bolometer temperature, which in turn changes in response to the quantity of absorbed incident infrared radiation. These characteristics can be exploited to measure incident infrared radiation on the bolometer by sensing the resulting change in its resistance. When used as an infrared detector, the bolometer is generally thermally isolated from its supporting substrate or surroundings to allow the absorbed incident infrared radiation to generate a temperature change in the bolometer material.
Microbolometer arrays are typically fabricated on monolithic silicon substrates or integrated circuits by constructing two-dimensional arrays of closely spaced air bridge structures coated with a temperature sensitive resistive material that absorbs infrared radiation. The air bridge structure provides thermal isolation between the microbolometer detector and the silicon substrate.
With each microbolometer functioning as a pixel within the array, a two-dimensional image or picture representation of the incident radiation can be generated by translating the changes in resistance of each microbolometer into a time-multiplexed electrical signal that can be displayed on a monitor or stored in a memory. The circuitry used to perform this translation is commonly known as the read out integrated circuit (ROIC), and may be fabricated as an integrated circuit in the silicon substrate. The microbolometer array may then be fabricated on top of the ROIC. The combination of the ROIC and microbolometer array is commonly known as a microbolometer infrared focal plane array (FPA).
Conventional microbolometers may have various performance limitations, such as for example decreased sensitivity due to inadequate thermal isolation, a temperature coefficient of resistance (TCR) mismatch of circuit elements, a contraction of optical absorption spectra, and/or a limited spectral response range. As a result, there is a need for improved detectors and manufacturing processes that may enhance detector performance.